Variable attenuation fo free-space light beams

ABSTRACT

One aspect is a method for controllably attenuating the beam of light ( 108 ) coupled between incoming and outgoing optical fibers ( 106 ) by misaligning minor surfaces ( 116   a,    116   b ) included of an optical switching module ( 100 ). Misalignment of the mirror surfaces ( 116   a  and  116   b ) causes only a portion of the beam of light ( 108 ) propagating along the incoming optical fiber ( 106 ), which is less than when the light beam deflectors&#39; mirror surfaces ( 116 ) are precisely aligned, to propagate along the outgoing optical fiber ( 108 ). Thus, the optical switching module ( 100 ) controllably attenuates the beam of light ( 108 ) coupled between the incoming and the outgoing optical fibers ( 106 ). Another aspect is a variable-optical-attenuator (“VOA”) ( 212 ) that includes an optically reflective membrane ( 222 ) upon which the beam of light ( 108 ) impinges. Application of an electrostatic field between an adjacent electrode ( 228 ) and the membrane ( 222 ) deforms the membrane ( 222 ) thereby attenuating an impinging beam of light ( 108 ).

TECHNICAL FIELD

[0001] The present invention relates generally to the technical field ofoptics and, more particularly, to attenuating a free-space light beam asmay propagate through a cross-connect fiber optic switch.

BACKGROUND ART

[0002] Attenuators of various different types are used throughoutcommunication equipment for adjusting the power level of carriersignals. Because optical amplifiers are becoming ubiquitous in fiberoptic systems for increasing the power level of optical carrier signals,variable optical attenuators are becoming increasingly important foradjusting the power level of optical communication signals. Suchvariable optical attenuators are particularly important for opticalcross-connect fiber optic switches because optical signals may arrive atthe optical switch from different places and therefore may havediffering signal strengths.

[0003] Patent Cooperation Treaty (“PCT”) international patentapplication WO 00/20899 published Apr. 13, 2000, entitled “Flexible,Modular, Compact Fiber Optic Switch,” (“the '899 PCT patentapplication”) describes an optical cross-connect for switchingquasi-collimated, free-space light beams. The '899 PCT patentapplication is hereby incorporated by reference as though fully setforth here. FIG. 1 illustrates one embodiment of an N×N opticalswitching module, indicated by the general reference character 100 anddescribed in the '899 PCT patent application, that may be included inthe fiber optic switch. The N×N optical switching module 100 includestwo arrays 118 a and 118 b of movable mirror surfaces 116 a and 116 b oflight beam deflectors that face each other.

[0004] As described in the '899 PCT patent application, each mirrorsurface 116 of the light beam deflectors is preferably provided by atwo-dimensional (“2D”) torsional scanner of a type similar to thosedescribed in U.S. Pat. No. 5,629,790 (“the '790 patent”), and in PCTinternational patent application WO 00/13210 published Mar. 9, 2000,entitled “Micromachined Members Coupled For Relative Rotation ByTorsional Flexure Hinges” (“the '210 PCT patent application”). Each 2Dtorsional scanner includes the mirror surface 116 which is coupled toand supported from an encircling frame by a first pair of hinges. Thefirst pair of hinges permit the mirror surface 116 to rotate about afirst axis with respect to the encircling frame. In turn, the encirclingframe of the torsional scanner is itself coupled to and supported froman outer reference frame by a second pair of hinges. The second pair ofhinges permit the encircling frame to rotate with respect to the outerreference frame about a second axis that is not oriented parallel to thefirst axis.

[0005] Each optical fiber 106 connected to the optical switching module100 in either of its two (2) sides 102 a and 102 b can direct a beam oflight 108 through a lens 112 to a unique entrance mirror surface 116 aor 116 b that is rotatable about the two non-parallel axescorrespondingly, each optical fiber 106 may also receive a beam of light108 that reflects from a unique exit mirror surface 116 a or 116 b. Eachentrance mirror surface 116 in one array 118 a or 118 b can be rotatedto point the beam of light 108 impinging thereon to any of the mirrorsurfaces 116 in the other array 118 b or 118 a. To couple a beam oflight 108 through the free-space between a pair of optical fibers 106,i.e. one optical fiber 106 respectively from each of the sides 102 a and102 b, the beam of light 108 from one of the optical fibers 106 in theside 102 a or 102 b impinges upon an entrance mirror surface 116 in thearray 118 a or 118 b, reflects off the entrance mirror surface 116 a or116 b to impinge upon a second exit mirror surface 116 b or 116 a in thearray 118 b or 118 a, and to then reflect therefrom into one of theoptical fibers 106 in the side 102 b or 102 a.

[0006] The loss of optical power in the beam of light 108 coupledbetween pairs of optical fibers 106 connected to the optical switchingmodule 100 depends critically on the respective orientations of the pairof mirror surfaces 116 a and 116 b in the light beam deflectors. Otherelements surrounding the optical switching module 100 may also increasethe amount of optical power loss.

[0007] To precisely align the orientations of the mirror surfaces 116 aand 116 b of the light beam deflectors, the fiber optic switch includesa dual axis servo controller 122 for each pair of mirror surfaces 116 aand 116 b that couple a beam of light 108 between a pair of opticalfibers 106. FIG. 2 illustrates one channel of the dual axis servocontroller 122.

[0008] As part of the dual axis servo controller 122, each optical fiber106 of the fiber optic cross-connect switch includes a directionalcoupler 124 for tapping off a fixed amount of the optical signal power,e.g. a 20 dB optical coupler. The optical signal extracted by-eachdirectional coupler 124 impinges upon a photo-detector 126. Eachphoto-detector 126 receives and measures the optical power present in afixed fraction of beam of light 108 propagating through the opticalswitching module 100 along the optical fibers 106 regardless of whetherthe optical fiber 106 is an incoming or an outgoing optical fiber 106.Precisely aligning the orientations of a pair of the mirror surfaces 116a and 116 b of the light beam deflectors causes as much as possible ofthe beam of light 108 emitted from the incoming optical fiber 106 topropagate along the outgoing optical fiber 106.

[0009] Between the directional coupler 124 on the incoming optical fiber106 and the optical switching module 100, and also past the directionalcoupler 124 on the outgoing optical fiber 106, there may exist otheroptical elements, such as additional couplers, switches, opticalamplifiers, connectors and cables, all of which contribute to loss (orgain) of optical signal power through the fiber optic switch. FIG. 2depicts the presence of these other optical elements respectively withthe loss elements 128 a and 128 b. Furthermore, in addition to the losselements 128 a and 128 b there may also exist loss elements, notillustrated in FIG. 2, which precede the directional coupler 124 on theincoming optical fiber 106, and are located between the opticalswitching module 100 and the directional coupler 124 on the outgoingoptical fiber 106.

[0010] The input and output power levels measured by the photo-detectors126 are supplied as input signals to the dual axis servo controller 122.The dual axis servo controller 122 uses these signals for properlyorienting the pair of mirror surfaces 116 a and 116 b. The dual axisservo controller 122 may implement various different servo controlalgorithms for controlling orientation of the mirror surfaces 116 a and116 b.

[0011] As stated above, optical signals may arrive at the opticalswitching module 100 via the optical fibers 106 from different placesand therefore may have differing signal strengths. Furthermore,differing wavelength optical signals may arrive at the optical switchingmodule 100 on differing optical fibers 106. Such multiple beams of lighthaving differing wavelengths, after passing through the opticalswitching module 100, may be multiplexed onto a single outgoing opticalfiber. If optical signals having differing signal strengths aremultiplexed together without controlling their respective strengths,wavelengths having different strength may increase differently duringsubsequent optical amplification. For this and other reasons it highlydesirable that all wavelengths being multiplexed into a single opticalfiber have approximately the same power.

[0012] In principle, such matching of the respective strengths of theoptical signal carried by a set of outgoing optical fibers 106 can beaccomplished by parsing each beam of light 108 through an attenuatorlocated between an incoming optical fiber 106 and the outgoing opticalfiber 106. However, because another fiber optic switch located elsewherein the telecommunication system can, at any time, switch an incomingoptical fiber 106 to a different optical signal source having adifferent signal strength, an attenuator included in the opticalswitching module 100 must be easily and quickly adjusted forappropriately attenuating optical signals of various strengths.

DISCLOSURE OF INVENTION

[0013] An object of the present invention is to provide a method foroperating a cross-connect fiber optic switch which permits controllablyattenuating a free-space beam of light propagating within the opticalswitching module.

[0014] Another object of the present invention is to provide an opticalattenuator that is easily controlled to provide differing amounts ofattenuation for a free-space beam of light.

[0015] Yet another object of the present invention is to provide simplevariable-optical-attenuator that us free standing, and that can also beeasily integrated into an array of variable-optical-attenuators.

[0016] Another object of the present invention is to provide avariable-optical-attenuator whose operation is independent of thewavelength of light impinging thereon.

[0017] Briefly, one aspect of the present invention is a method foroperating a fiber optic switch for controllably attenuating the beam oflight that the fiber optic switch couples between an incoming and anoutgoing optical fiber. The method for controllably attenuating the beamof light includes supplying to the servo controller a control signalwhich causes the servo controller to misalign mirror surfaces of theoptical switching module's light beam deflectors. The misalignment ofthe light beam deflectors' mirror surfaces causes the optical switchingmodule to couple into the outgoing optical fiber only a portion of thebeam of light propagating along the incoming optical fiber which is lessthan when the light beam deflectors' mirror surfaces are preciselyaligned. In this way the fiber optic switch controllably attenuates thebeam of light coupled between the incoming and the outgoing opticalfibers.

[0018] Another aspect of the present invention is avariable-optical-attenuator (“VOA”) for attenuating a beam of light thatincludes an optically reflective membrane upon which the beam of lightmay impinge. The VOA also includes an insulating substrate across whichthe membrane is secured. Secured in this location, the membrane isseparated a suitable distance from a surface of the substrate upon whichan electrode is disposed. Application of an electrostatic field betweenthe membrane and the underlying electrode deforms the membrane.Deformation of the membrane causes an impinging beam of light to beattenuated due to aberrations in the beam of light reflected from themembrane. For use in cross-connect fiber optic switches, these VOAs canbe arranged into 16×16, 64×64 or 256×256 arrays.

[0019] These and other features, objects and advantages will beunderstood or apparent to those of ordinary skill in the art from thefollowing detailed description of the preferred embodiment asillustrated in the various drawing figures.

BRIEF DESCRIPTION OF DRAWINGS

[0020]FIG. 1 is a is a plan view ray tracing diagram illustratingpropagation of light beams through a free-space N×N reflective switchingmodule of a fiber optic switch;

[0021]FIG. 2 is a block diagram illustrating a servo controller includedin the fiber optic switch for controlling orientations of light beamdeflectors' mirror surfaces included in the optical switching module;

[0022]FIG. 3a graphically illustrates optical power coupled through aoptical switching module as a function of drive signals which effectrotation about the two non-parallel axes;

[0023]FIG. 3b graphically illustrates optical power coupled through aoptical switching module, similar to the illustration of FIG. 3a, as afunction of drive signals adjusted for the light beam deflector'ssensitivity about each of its rotation axes;

[0024]FIG. 4 is a graph illustrating the difference in attenuation whichoccurs due to pure rotation of a mirror surface, and that due totruncation of the optical beam because a portion thereof misses themirror surface which reflects the beam to the outgoing optical fiber;

[0025]FIG. 5 is a waveform, diagram illustrating attenuation of anoptical signal after switching thereof effected by the servo controller;

[0026]FIG. 6 is a block diagram depicting an optical circuit thatincludes a deformable-membrane, reflective variable-optical-attenuatorin accordance with the present invention;

[0027]FIG. 7a is a plan view of one embodiment of a deformable-membrane,reflective variable-optical-attenuator in accordance with the presentinvention;

[0028]FIG. 7b is a cross-sectional, elevational view of thedeformable-membrane, reflective variable-optical-attenuator taken alongthe line 7 b-7 b of FIG. 7a;

[0029]FIG. 8 is a diagram illustrating attenuation of a light beam thatincreasing deformation of a reflective membrane provides;

[0030]FIG. 9 is a plan view of an alternative embodiment for adeformable-membrane, reflective variable-optical-attenuator inaccordance with the present invention;

[0031]FIGS. 10a through 10 c are plan views of various different flexureconfigurations, taken along the line 10 a/10 c-10 a/106 in FIG. 9, asmay be incorporated into the deformable-membrane, reflectivevariable-optical-attenuator depicted in that FIG.;

[0032]FIGS. 11a and 11 b are plan views illustrating yet otheralternative embodiments for the deformable-membrane, reflectivevariable-optical-attenuator that require a lesser amount of deflection;and

[0033]FIG. 12 is a block diagram depicting how an individualdeformable-membrane, reflective variable-optical-attenuator inaccordance with the present invention may be simply included in areflective switching module adapted for inclusion in a opticalcross-connect fiber optic switch; and

[0034]FIG. 13 is a block diagram depicting how an array ofdeformable-membrane, reflective variable-optical-attenuators may besimply included in a reflective switching module adapted for inclusionin a optical cross-connect fiber optic switch.

BEST MODE FOR CARRYING OUT THE INVENTION

[0035] Mirror Misalignment Attenuation

[0036] One aspect of the present invention uses misalignment fromoptimum orientations for the pair of mirror surfaces 116 a and 116 b forattenuating the beam of light 108 coupled between a pair of opticalfibers 106 by the optical switching module 100 such as that depicted inFIG. 1. In a most basic, first operating mode for the fiber opticswitch, i.e. a minimum loss operating mode, the dual axis servocontroller 122 precisely aligns the pair of mirror surfaces 116 a and116 b to couple the maximum amount of optical signal power, i.e. light,between the pair of optical fibers 106.

[0037] The amount of light coupled between a pair of optical fibers 106by two mirror surfaces 116 a and 116 b varies approximately as aGaussian function 142 as one or the other of the pair of mirror surfaces116 rotates about either one or the other of the torsional scanner'stwo, non-parallel axes. However, for the same driving voltage applied toan electrostatically energized torsional scanner, the width of theGaussian function 142 differs depending upon the axis about which themirror surface 116 rotates. FIG. 3a graphically illustrates opticalsignal power coupled through the optical switching module 100 for theexit mirror surface 116 for driving signals applied to the torsionalscanner which rotate the exit mirror surface 116 about either one or theother of the two non-parallel axes. Rotation of the entrance mirrorsurface 116, discussed in greater detail below, produces a very similarcurve to that depicted in FIG. 3a.

[0038] As graphically depicted in FIG. 3a, the Gaussian function 142differs for rotation about each of the axes because the mirror surface116 rotates through a larger angle about either one or the other of theaxes for the same voltage applied to independently energize rotation ofthe mirror surface 116 about each of the non-parallel axes. Differingamounts of rotation of the mirror surface 116 occur about the two axesas graphically depicted in FIG. 3a because the hinges supporting themirror surface 116 for rotation about one axis are stiffer than thehinges supporting the mirror surface 116 for rotation about the otheraxis. Properly compensating the voltages applied to the torsionalscanner for energizing rotation of the mirror surface 116 about each ofthe axes for the respective stiffness of the hinges produces a Gaussianfunction 144 such as that depicted in FIG. 3b. Due to the four rotationaxes that are involved in coupling the beam of light 108 between thepair of optical fibers 106 (i.e. two axes for the mirror surface 116 aor 116 b and two axes for the mirror surface 116 b or 116 a), theGaussian function 144 of FIG. 3b must be understood to be afour-dimensional function which typically exhibits differing responsesalong at least two of its four axes.

[0039] It is noteworthy that due to the steep slope of the Gaussianfunction 142 or 144 when moving away from its peak where the mirrorsurfaces 116 are precisely aligned to couple the maximum amount ofoptical signal power between the optical fibers 106, coupling of lightbetween the optical fibers 106 by the optical switching module 100becomes much more sensitive to small angular rotations of the mirrorsurfaces 116 a and 116 b. Hence, when using mirror surfaces 116 a and116 b it is advantageous to select for attenuating the beam of light 108that particular axis of the four axes which exhibits the leastsensitivity to rotation of the mirror surface 116. This is true whetherthe entrance mirror surface 116 is being rotated to attenuate the beamof light 108, or if the exit mirror surface 116 is being rotated toattenuate the beam of light 108. Experimentally it has been observedthat a rotation of the mirror surface 116 about a less sensitive axisproduces much less noise in the output optical signal received by theoutgoing optical fiber 106 than a rotation, that produces the samereduction in output power, about a more sensitive axis. Hence, inattenuating optical signals using misalignment of the mirror surfaces116 a and 116 b it is generally preferable to rotate the mirror surface116 about the least sensitive of the four axes.

[0040] If the mirror surfaces 116 were infinitely large in comparisonwith the diameter of the impinging quasi-collimated beam of light 108,the angular response in output power coupled between the pair of opticalfibers 106 would be the same for both of the mirror surfaces 116 a and116 b for rotation axes that are equivalently stiff. However, forsmaller mirror surface 116 having a size that is approximately equal tothat of the impinging quasi-collimated beam of light 108, rotation ofthe entrance mirror surface 116 rapidly causes a portion of the beam oflight 108 to miss the exit mirror surface 116 a or 116 b. Suchvignetting of the beam of light 108 will, of course, also affect theamount of optical signal power coupled through the optical switchingmodule 100 but in a different way than that described above with respectto FIGS. 3a and 3 b.

[0041] A solid curve 152 in FIG. 4 graphically depicts the effect ofvignetting on the transmitted optical signal as a function of rotationof the entrance mirror surface 116. The curve 152 illustrates the effectof vignetting for a optical switching module 100 in which the mirrorsurfaces 116 a and 116 b are spaced 500 mm apart, the mirror surfaces116 present an approximately 1.5 mm square surface area to the impingingbeam of light 108, and the lenses 112 have a 6 mm focal length. For sucha configuration of the optical switching module 100, the curve 152depicts rotation of the entrance mirror surface 116 while the exitmirror surface 116 is held in its optimum orientation. The curve 152shows that if only small adjustments are required to effect a desiredattenuation of the beam of light 108, then vignetting provides a ratherbroad top for the curve 152 which facilitates misaligning the exitmirror surface 116 to obtain the desired attenuation. However, forlarger angles the optical power coupled into the outgoing optical fiber106 drops precipitously, and controlling attenuation becomes moredifficult. If the mirror surfaces 116 a and 116 b are located physicallycloser to each other, the shape of the curve 152 will differ from thatdepicted in FIG. 4.

[0042] A dashed curve 154 in FIG. 4 graphically depicts the transmittedoptical signal directly from a misaligned exit mirror surface 116 whilethe entrance mirror surface 116 is held in its optimum orientation. Thissecond method for attenuating the beam of light 108 avoids vignetting ofthe beam of light 108. From the illustration of FIG. 4, it is readilyapparent that rotations of the mirror surface 116 as small as one-half amilli-radian markedly reduces the optical power in the beam of light 108received by the outgoing optical fiber 106, i.e. milli-radian rotationsdramatically affect attenuation of the beam of light 108 leaving theoptical switching module 100.

[0043] Apart from the preceding geometric optical considerations whichinfluence which one of a pair of mirror surfaces 116 is preferablymisaligned to attenuate the beam of light 108, there exist otherconsiderations about torsional scanner operation that may prohibitselecting a particular axis of a particular mirror surface 116. Forexample, if rotation of the mirror surface 116 places the torsionalscanner in the unstable electrostatic operating range for a particularaxis of rotation, then it may be preferable to avoid using misalignmentabout that axis for attenuating the beam of light 108. An axis aboutwhich rotation of the 116 is in the unstable electrostatic operatingrange is prone to more instability. Thus, rotation about such anunstable axis will introduce more noise into the optical signal receivedby the outgoing optical fiber 106 than if one of the mirror surfaces 116were servoed on a portion of the Gaussian functions 142 and 144 or thecurves 152 and 154 having a gentler slope. Also, one of the rotationaxes of one of the mirror surface 116 may inherently have lowerpositional noise, for example because it has a lower resonant frequencyor experiences greater fluid damping from atmosphere surrounding thetorsional scanner.

[0044] The two curves 152 and 154 together with the precedingdescription of FIGS. 3a and 3 b demonstrate that selecting a particularaxis of one of the pair of mirror surfaces 116 a and 116 b formisalignment to attenuate the beam of light 108 depends upon a varietyof considerations including the particular pair of optical fibers 106between which light is being coupled, the distance between andorientation angles of the mirror surfaces 116 a and 116 b, and even theamount of attenuation required. Consequently, selecting the best methodfor attenuating the beam of light 108 may be performed dynamicallyduring operation of the fiber optic switch.

[0045] In general, one, two, three or all four axes of rotation may bemisaligned from their respective optimum orientations to obtain adesired attenuation. Since there exist essentially an infinite number ofconfigurations for the pair of mirror surfaces 116 in the fourdimensional space described above, in general, at any instant in timethere exist many different configurations that could be adapted toproduce a particular desired attenuation.

[0046] Note that in general it is preferable to employ smallmisalignments around each of the four axes rather than a single largerrotation because the slope of the Gaussian function is less precipitousfor small rotations. Small misalignments around each of the four axesproduces the same total attenuation by summing the four, individualsmaller attenuations. Using small misalignments around each of the fouraxes increases stability of the attenuation, and therefore the opticalsignal propagating along the outgoing optical fiber 106 exhibits lessnoise.

[0047] Initially, the optimum mirror positions are determined in whichthe pair of mirror surfaces 116 are precisely aligned. Usually, thereexists only one set of orientations for the mirror surfaces 116 thatproduces the configuration in which the optical switching module 100causes as much as possible of the beam of light 108 emitted from theincoming optical fiber 106 to propagate along the outgoing optical fiber106, i.e. an optimum power throughput position. Then, given theattenuation needed for matching of the respective strengths of theoptical signal carried by a set of outgoing optical fibers 106, acalculation is performed which determines the amount of rotation about aparticular axis for a particular mirror surface 116 that is required tomisalign the pair of mirror surfaces 116 to obtain the specifiedattenuation of the beam of light 108. A signal specifying themisalignment is then supplied to the appropriate dual axis servocontroller 122 to effect the specified rotation of the mirror surface116 from its optimum orientation. The optimum orientations required forthe pair of mirror surfaces 116 about the three remaining axes are thenalso transmitted to the dual axis servo controllers 122 whichrespectively control rotation about those axes. The axis beingmisaligned to produced the desired attenuation may be servoed at aslower rate to maintain the attenuation.

[0048]FIG. 5 presents an actual oscilloscope trace 162 from thedirectional coupler 124 where switching and subsequent attenuation ofthe optical signal received by the outgoing optical fiber 106 occursbeginning at a point 164. The behavior exhibited in the oscilloscopetrace 162 of FIG. 5 has been observed during operation of a opticalswitching module 100 that were first made in March 1999, using 2D mirrorsurfaces 116 whose orientations were controlled by analog dual axisservo controllers 122.

[0049] Deformable Reflective Attenuator

[0050]FIG. 6 depicts an optical circuit, referred to by the generalreference character 200, that is adapted to include adeformable-membrane, -reflective VOA 212 in accordance with the presentinvention. In the illustration of FIG. 6, similar to the opticalswitching module 100 depicted in FIG. 1, the optical circuit 200includes an incoming optical fiber 202 which emits a beam of light thatimpinges on a lens 204. The lens 204 is disposed with respect to an end206 of the incoming optical fiber 202 to produce from light emitted fromthe end 206 a quasi-collimated beam of light 208. The beam of light 208propagates horizontally in the illustration of FIG. 6 through theoptical circuit 200 to impinge upon a reflective VOA 212. The beam oflight 208 reflects off the VOA 212 to continue propagating through theoptical circuit 200 vertically downward in the illustration of FIG. 6until impinging upon a lens 214. The lens 214 focuses the impinging beamof light 208 onto an end 216 of an outgoing optical fiber 218.

[0051] As illustrated in FIGS. 7a and 7 b, the VOA 212 in accordancewith the present invention is formed as a disk-shaped, reflectivemembrane 222. The membrane 222 may be a few microns to severalmillimeters (“mm”) thick, is suitably formed in a silicon-on-insulator(“SOI”) wafer in a manner similar to that described in U.S. Pat. Nos.5,488,862, 5,629,790 and 6,044,705, and then subsequently coated withgold or any other suitably reflective material. U.S. Pat. Nos.5,488,862, 5,629,790 and 6,044,705 are hereby incorporated by referenceas though fully set forth here. Alternatively, the membrane 222 may befabricated from other suitably materials such as nitrides, oxides,oxynitrides or metals.

[0052] As depicted in FIG. 7b, the membrane 222 is secured to aninsulating substrate 224 by spacers 226. The spacers 226 hold themembrane 222 a suitable distance above an electrode 228 that is coatedonto a surface of the substrate 224 adjacent to the membrane 222. Thesubstrate 224 may be made of silicon as may be the membrane 222 whichmay, if necessary, be formed integrally as part of the substrate 224.

[0053] Applying an electrostatic force between the membrane 222 and theelectrode 228 deforms the membrane 222 as indicated in FIG. 7b. When theVOA 212 is positioned in the optical circuit 200 as depicted in FIG. 6,deformation of the membrane 222 causes aberration in the beam of light208 that impinges upon the VOA 212. This aberration in the beam of light208 attenuates transmission of light from the incoming optical fiber 202to the outgoing optical fiber 218. A curve 232 in FIG. 8 graphicallydepicts the type of attenuation provided by a membrane 222 which has aradius 1.5×NA×f, where NA is the numerical aperture of the lenses 204and 214 (e.g. 0.15) and f the focal. As illustrated in FIG. 2,relatively small deformations of the membrane 222 can substantiallyattenuate the beam of light 208. For example, a 2.0 micron thick siliconmembrane 222 that is 2.0 mm in diameter will deform 15.0 microns when auniform electrostatic field of 5.0 volts/micron is applied between themembrane 222 and the electrode 228. For precise attenuation control,stress sensors may be integrated into the silicon membrane so a desireddeformation can be detected electronically.

[0054] The sensitivity of the VOA 212 can be increased by subdividingthe membrane 222 into a nested, concentric set of annularly-shapedmembranes 242 a-242 d as illustrated in FIG. 9. Very narrow slits 244separate the annularly-shaped membranes 242 a-242 d so that in theundeformed state the composite membrane 222 is essentially flat, and theslits 244 produce very little scattering from the surface of thecomposite membrane 222. The annularly-shaped membranes 242 a-242 d areinterconnected by narrow flexures 246 which allow the annularly-shapedmembranes 242 a-242 d making up the composite membrane 222 to deformmore readily in response to an applied electrostatic force. Thecomposite membrane 222 depicted in FIG. 9 exhibits more deformation forthe same applied electrostatic field than the membrane 222 depicted inFIGS. 7a and 7 b, or equal deformation for a much lower electrostaticfield.

[0055]FIGS. 10a through 10 c depict various different configurations forthe flexures 246 which join immediately adjacent annularly-shapedmembranes 242 to each other. The flexures 246 can be easily etched intothe membrane leaving a minimum of open area between immediately adjacentannularly-shaped membranes 242. Because the composite membrane 222deforms readily in an approximately Gaussian shape, the deformation of afull circular, composite membrane 222 is much more than a wavelength ofthe beam of light 208 impinging thereon.

[0056]FIG. 11a illustrates a VOA 212 having a slit membrane 222 in whichtwo semicircular halves 252 a and 252 b two halves 252 a and 252 b areseparated by a narrow gap 254 along a diameter of the circularly-shapedmembrane 222. The electrode 228 on the immediately adjacent substrate224 underlies only the semicircular half 252 b. The substrate 224underlying the semicircular half 252 a lacks the electrode 228, andtherefore the semicircular half 252 a remains flat even though thesemicircular half 252 b deforms in response to an applied electrostaticforce. Hence by deforming the semicircular half 252 b, a phase shift canbe created between two halves of the beam of light 208 that impinges onthe membrane 222. Such a phase shift between two halves of the beam oflight 208 produces substantial diffraction, and hence reduces couplingof the beam of light 208 between the incoming optical fiber 202 and theoutgoing optical fiber 218. To obtain substantial diffraction in thebeam of light 208, the semicircular half 252 b need deform approximatelyone-quarter of the wavelength of light in the beam of light 208 withrespect to the semicircular half 252 a. For the VOA 212 illustrated inFIG. 11a, attenuation of the beam of light 208 is maximized when theseparation between the semicircular half 252 a and the semicircular half252 b is an integral multiple of one quarter wavelength of light in thebeam of light 208.

[0057] The principle embodied in the VOA 212 illustrated in FIG. 11a canbe extended to multiple sections 262 as illustrated in FIG 11 b adjacentto which electrodes 228 are disposed on the substrate 224. In the VOA212 illustrated in FIG. 11b, each of the sections 262 may beindividually deformed while all other portions of the composite membrane222 that are located between and adjacent to the sections 262 remainflat. Thus, each section 262 may be individually deformed as desiredcausing each of the sections 262 to individually and independentlydistort the beam of light 208 impinging thereon. In contrast to the VOA212 illustrated in FIG. 11a, deformation of the sections 262 need not beapproximately one-quarter of the wavelength of light to significantlyattenuate the beam of light 208.

[0058] It is readily apparent that other subdivisions of thecircularly-shaped membrane 222 in addition to those illustrated in FIGS.11a and 11 b are possible. However, the subdivisions illustrated inFIGS. 9, 11a and 11 b sufficiently exemplify the concepts embodied insuch VOAs 212. Furthermore, all of the subdivisions depicted in FIGS. 9,11a and 11 b provide simple mechanical support at the periphery of themembrane 222 for the annularly-shaped membranes 242 a-242 d, the twohalves 252 a and 252 b, and the sections 262 which is important for easein fabricating and assembling the VOA 212.

[0059] For some embodiments of the VOAs 212, particularly thoseillustrated in FIGS. 11a and 11 b, it is advantageous if any coating ofgold or other suitable reflective material be applied in a way thatreduces the possibility of creating unbalanced stresses and hencedeformation on the membrane 222. U.S. Pat. No. 6,044,705 entitled“Micromachined Members Coupled For Relative Rotation By Torsion Bars”that issued Apr. 4, 2000 (“the '705 patent”), and the '210 PCT patentapplication describe procedures for applying such a reflective coatingthat reduces the possibility of creating unbalanced stresses on themembrane 222. Both the '705 patent and the '210 PCT patent applicationare hereby incorporated by reference as though fully set forth here.

[0060] As described above, the beam of light 108 propagating within theoptical switching module 100 between a pair of optical fibers 106 can beattenuated by misaligning one or both of the pair of mirror surfaces 116from their precisely aligned orientations. However, as described aboveobtaining significant amounts of attenuation in this way is difficultbecause it requires that the dual axis servo controller 122 maintaineach misaligned mirror surface 116 precisely at its specifiedorientation. Each misaligned mirror surface 116 must be maintainedprecisely at its specified orientation because, as graphicallyillustrated in FIGS. 3a, 3 b and 4, for large attenuations the amount oflight coupled between the incoming and the outgoing optical fiber 106changes precipitous for a slight change in the orientation of the mirrorsurface 116.

[0061]FIG. 12 depicts how any of the VOAs 212 depicted in FIGS. 7a and 7b, 9, 11 a and 11 b may be incorporated into a reflective switchingmodule of the type included in a fiber optic switch. In the illustrationof FIG. 12, the incoming optical fiber 106 directs the beam of light 108through the lens 112 to impinge upon the VOA 212. The beam of light 108reflecting off the membrane 222 impinges upon the entrance mirrorsurface 116 arranged with respect to the VOA 212 in a configurationsimilar to that depicted in FIG. 21 of the '899 PCT patent application,and described in the text thereof for that FIG. In a reflectiveswitching module, the VOAs 212 and the mirror surfaces 116 may bearranged in vertical columns as described in the '899 PCT patentapplication with the fixed mirror depicted in FIG. 31 being replaced bythe array of VOAs 212, one VOA 212 for each beam. In general, a singleVOA 212 located in the free-space optical path between the incoming andoutgoing optical fibers 106 provides sufficient attenuation for mostapplications of the fiber optic switch. However, the side 102 to whichoutgoing optical fibers 106 connect may, if necessary to obtainadditional attenuation, also include similar VOAs 212 that are arrangedin the configuration depicted in FIG. 21 of the '899 PCT patentapplication.

[0062] The arrangement of the VOAs 212 and the mirror surfaces 116depicted in FIG. 12 lends itself to using a bank 282 of lenses 112 and abank 284 of entrance mirror surfaces 116 as illustrated in FIG. 13. Anarray 286 of VOAs 212, interposed between the bank 282 and the bank 284,provides any desired attenuation of the beams of light 108. For example,the lenses 112 may be arranged in a 16×16, 64×64 or 256×256 bank 282which directs individual beams of light 108 to individual membranes 222in the corresponding 16×16, 64×64 or 256×256 array 286 of VOAs 212. Suchbanks of lenses 112 and mirror surfaces 116, and arrays of VOAs 212 maybe used either or both for incoming and outgoing optical fibers 106.

INDUSTRIAL APPLICABILITY

[0063] Based on a particular application for the fiber optic switch, theoptical switching module 100 can operate concurrently in severaldifferent modes for individual pairs of optical fibers 106 as describedin greater detail below. While operating modes for attenuating opticalsignals is described below in the context of misaligning pairs of mirrorsurfaces 116, these operating modes may in fact also be implementedusing the VOAs 212 also described above.

[0064] 1. Minimum Loss Mode. Operating in this way, the opticalswitching module 100 couples into the outgoing optical fiber 106 as muchas possible of the beam of light 108 emitted from the incoming opticalfiber 106. This type of connection between pairs of optical fibers 106provides optimum power transmission from input to output of the opticalswitching module 100. This operating mode can be used with all or someof the pairs of optical fibers 106 connected to the optical switchingmodule 100 in conjunction with mode 2 and mode 3, described in greaterdetail below, for other pairs of optical fibers 106.

[0065] 2. Fixed Loss Mode. Operating in this way, the attenuationimposed on the optical signal being coupled through the opticalswitching module 100 between pairs of optical fibers 106 is made thesame, or equalized, for each selected pair of incoming and outgoingoptical fibers 106. In mode 2, the dual axis servo controller 122adjusts the pair of mirror surfaces 116 so the ratio of the output toinput optical signal power levels is maintained at a fixed, desiredvalue. If the optical signal power received by the outgoing opticalfiber 106 is too high, the dual axis servo controller 122 reorients oneor both of the mirror surfaces 116 to adjust the attenuation therebyrestoring the ratio of output to input power levels. This operating mode2 can be used to equalize attenuations among a group of optical signalconnections thereby minimizing variation in attenuation of the opticalsignals passing through the optical switching module 100. For example,if the attenuation between incoming optical fiber 106 X and outgoingoptical fiber 106 Z is 5 dB, then alignment of the mirror surfaces 116for coupling between incoming optical fiber 106 X and outgoing opticalfiber 106 Y, which if unattenuated could be 3 dB, can be misaligned toestablish a matching 5 dB attenuation. An attenuation table may beproduced during manufacture and calibration of the optical switchingmodule 100, and the attenuation in the signal path may be adjusted uponestablishing a connection between a particular pair of optical fibers106. In addition, the actual attenuation in the coupling between thepair of optical fibers 106 may be measured in real-time, and theattenuation may be appropriately adjusted during operation of the fiberoptic switch. This operating mode 2 can be used with all or some of thepairs of optical fibers 106 connected to the optical switching module100 in conjunction with mode 1 and mode 3 applied to other pairs ofoptical fibers 106.

[0066] 3. Fixed Output Power Mode. Operating in this way, the power inthe optical signal being coupled into the outgoing optical fiber 106 ismaintained as close as possible to a desired, pre-established powerlevel. In mode 3, ignoring the input power level the dual axis servocontroller 122 orients the pair of mirror surfaces 116 to maintain afixed power level in the beam of light 108 received by the outgoingoptical fiber 106. This operating mode could be used to match powerlevels in several beams of light 108 respectively coupled into an equalnumber of outgoing optical fibers 106. Matching power levels in thebeams of light 108 coupled into outgoing optical fibers 106 is desirableif several different wavelengths of light that pass through the opticalswitching module 100 are to be multiplexed together into a singleoptical fiber 106 before optical amplification. This mode of operationcan also be used to reestablish the power level of signals passingthrough the optical switching module 100 to a nominal value. Again thisoperating mode can be used with all or some of the pairs of opticalfibers 106 connected to the optical switching module 100 in conjunctionwith mode 1 and mode 2 used for other pairs of optical fibers 106.

[0067] It should be noted that all three of the preceding operatingmodes may exist concurrently for differing pairs of optical fibers 106in various parts of the optical switching module 100. Differentattenuations or output power levels may also be specified for particularmirror surfaces 116 or particular pairs of mirror surfaces 116. Itshould be understood that these various operating modes may be usedsimultaneously on different parts of the optical switching module 100,i.e. some pairs of optical fibers 106 may operate in mode 1, others inmode 2, and yet others in mode 3, depending upon the destination andfunction of the optical signal received by the outgoing optical fiber106.

[0068] It should further be noted that it is possible to compensate atthe optical switching module 100 for additional variations inattenuation that may occur in equipment further along atelecommunication path. Additionally, the fiber optic switch couldreceive in real-time, through a network management signaling system,power measurements from equipment that is located further along thetelecommunication path, and then use such power measurements todynamically adjust the attenuation through the optical switching module100 thus providing optimal network performance.

[0069] Although the present invention has been described in terms of thepresently preferred embodiment, it is to be understood that suchdisclosure is purely illustrative and is not to be interpreted aslimiting. Consequently, without departing from the spirit and scope ofthe invention, various alterations, modifications, and/or alternativeapplications of the invention will, no doubt, be suggested to thoseskilled in the art after having read the preceding disclosure.Accordingly, it is intended that the following claims be interpreted asencompassing all alterations, modifications, or alternative applicationsas fall within the true spirit and scope of the invention.

What is claimed is:
 1. A method for operating a fiber optic switch forcontrollably attenuating a beam of light that the fiber optic switchcouples between an incoming optical fiber and an outgoing optical fiberthat are both connected to the fiber optic switch, the fiber opticswitch including: a. a optical switching module within which propagatesa free-space beam of light emitted from the incoming optical fiber, theoptical switching module including light beam deflectors whichselectively couple the free-space beam of light emitted from theincoming optical fiber into a specific one of a plurality of opticalfibers which, upon receiving the beam of light, becomes the outgoingoptical fiber; and b. a servo controller which has a first operatingmode that precisely aligns the light beam deflectors so the opticalswitching module causes as much as possible of the beam of light emittedfrom the incoming optical fiber to propagate along the outgoing opticalfiber; the method for controllably attenuating the beam of lightcomprising the step of supplying to the servo controller a controlsignal which causes the servo controller to misalign the light beamdeflectors so the optical switching module causes only a portion of thebeam of light propagating along the incoming optical fiber, which isless than when the light beam deflectors are precisely aligned, topropagate along the outgoing optical fiber, whereby the fiber opticswitch controllably attenuates the beam of light coupled between theincoming and the outgoing optical fibers.
 2. The method of claim 1wherein the control signal supplied to the servo controller actuates asecond operating mode of the fiber optic switch in which the portion ofthe beam of light propagating along the outgoing optical fiber has afixed ratio to the beam of light propagating along the incoming opticalfiber.
 3. The method of claim 2 wherein the control signal supplied tothe servo controller actuates yet another operating mode of the fiberoptic switch which maintains a pre-established power level in the beamof light propagating along the outgoing optical fiber.
 4. The method ofclaim 1 wherein the control signal supplied to the servo controlleractuates yet another operating mode of the fiber optic switch whichmaintains pre-established power level in the beam of light propagatingalong the outgoing optical fiber.
 5. The method of claim 1 wherein thelight beam deflectors may be misaligned about several independent axes,and wherein the control signal supplied to the servo controller causesthe light beam deflectors to be appropriately misaligned about all axesthereby increasing stability of attenuation of the beam of light.
 6. Avariable-optical-attenuator (“VOA”) for attenuating a beam of lightcomprising: an optically reflective membrane adapted for impingement ofa beam of light thereon; and an insulating substrate across which saidmembrane is secured while being separated a suitable distance from asurface of said substrate upon which an electrode is disposed thatunderlies said membrane, whereby upon application of an electrostaticfield between said membrane and said electrode said membrane deforms anda beam of light impinging thereon is attenuated due to aberrations inthe beam of light reflected from said membrane.
 7. The VOA of claim 6wherein said membrane is disk-shaped.
 8. The VOA of claim 6 wherein saidmembrane is composite being formed by a concentric set ofannularly-shaped membranes with flexures joining immediately adjacentpairs of the annularly-shaped membranes.
 9. The VOA of claim 6 whereinsaid membrane is composite being formed by two semi-circular half disksthat are juxtaposed with each other on opposite sides of a diametricalgap.
 10. The VOA of claim 9 wherein the electrode underlies only one ofthe semi-circular half disks which form the composite membrane.
 11. TheVOA of claim 6 wherein said membrane is composite being formed byseveral sections between which are other portions of the membrane. 12.The VOA of claim 11 wherein the electrode underlies only the sections ofthe composite membrane.
 13. A method for operating a fiber optic switchfor controllably attenuating a beam of light that the fiber optic switchcouples between an incoming optical fiber and an outgoing optical fiberthat are both connected to the fiber optic switch, the fiber opticswitch including: a. a optical switching module within which propagatesa free-space beam of light emitted from the incoming optical fiber, theoptical switching module including: i) light beam deflectors whichselectively couple the free-space beam of light emitted from theincoming optical fiber into a specific one of a plurality of opticalfibers which, upon receiving the beam of light, becomes the outgoingoptical fiber; ii) a VOA having: A) an optically reflective membranedisposed so the beam of light impinges thereon; and B) an insulatingsubstrate across which the membrane is secured while being separated asuitable distance from a surface of the substrate upon which anelectrode is disposed, whereby upon application of an electrostaticfield between the membrane and the electrode the membrane deforms andthe beam of light impinging thereon is attenuated due to aberrations inthe beam of light reflected from the membrane; and b. a servo controllerwhich has a first operating mode that precisely aligns the light beamdeflectors so the optical switching module causes as much as possible ofthe beam of light emitted from the incoming optical fiber to propagatealong the outgoing optical fiber; the method for controllablyattenuating the beam of light comprising the step of supplying to theVOA a control signal which causes the VOA to deform so the opticalswitching module causes only a portion of the beam of light propagatingalong the incoming optical fiber, which is less than when the light beamdeflectors are precisely aligned, to propagate along the outgoingoptical fiber, whereby the fiber optic switch controllably attenuatesthe beam of light coupled between the incoming and the outgoing opticalfibers.
 14. The method of claim 13 wherein the control signal suppliedto the VOA actuates a second operating mode of the fiber optic switch inwhich the portion of the beam of light propagating along the outgoingoptical fiber has a fixed ratio to the beam of light propagating alongthe incoming optical fiber.
 15. The method of claim 14 wherein thecontrol signal supplied to the VOA actuates yet another operating modeof the fiber optic switch which maintains a pre-established power levelin the beam of light propagating along the outgoing optical fiber. 16.The method of claim 13 wherein the control signal supplied to the VOAactuates yet another operating mode of the fiber optic switch whichmaintains pre-established power level in the beam of light propagatingalong the outgoing optical fiber.
 17. An improved fiber optic switchthat couples beams of light between selected incoming and outgoingoptical fiber pairs that are connected to the fiber optic switch, thefiber optic switch including: a. a optical switching module within whichpropagates free-space beams of light emitted from incoming opticalfibers, the optical switching module including a bank of light beamdeflectors from which reflect the free-space beams of light emitted fromincoming optical fiber; and b. a servo controller for precisely aligninglight beam deflectors so the optical switching module couples into theoutgoing optical fiber of each pair as much as possible of the beam oflight emitted from the incoming optical fiber of the pair; wherein theimprovement comprises: an array of VOAs each of which includes: a. anoptically reflective membrane disposed so one of the beams of lightimpinges thereon; and b. an insulating substrate across which themembrane is secured while being separated a suitable distance from asurface of the substrate upon which an electrode is disposed, wherebyupon application of an electrostatic field between the membrane and theelectrode the membrane deforms and the beam of light impinging thereonis attenuated due to aberrations in the beam of light reflected from themembrane.